I totally agree that all contacts are not created equal.
The contact themselves would have a reliability rating that would be at first unknown to the player. as they deal with them more they will get a feel for how reliable they are. Information is really important in most rpgs but especially in a campaign like mine that has spies, traitors, turncoats and plenty of rumours, part truths and misinformation passing hands.
A devious NPC could plant a 'reliable' contact in the pcs path and after a time feed them misinformation or simply use them as a spy to keep tabs on the parties activities [cue evil GM laugh here]
I might even go as far to have a 3rd category of contact called either ORGANISATION, GUILD or FACTION.
This sort of contact could be used to find out information specific to a topic the guild specialised in (eg. need a rare poison? weapons? or information about the high ranking official that just arrived at the docks? your assassin's guild contact will should be able to help you here... of course they might also want to know why you need the information
Each contact should also have a proficiency rating such as:
POOR = 20%
AVERAGE = 40%
GOOD = 60%
EXCELLENT = 80%
This would be the the chance that contact would have information or a whatever the player was looking for. as i mentioned above, this rating would not be known to the player at first, but after a number of encounters with them they would get a feel for how useful they were.
I dont mind that different professions have cheap streetwise skill cost. That burglar pc should have access to more contacts in an urban setting than say a paladin or magician who would still have contacts but most of these would be linked to their guild/temple/faction eg. the mage has an alchemist (good) contact and a scribe (excellent) contact who are a good/excellent source of info/resources about rings/scrolls.
Compare this to the burglar who might have thieves guild contact (Good) and a fence named Jake (Poor) as well as an outfitter name Saul (Excellent) a retired rogue who is a great source for rumours about possible 'jobs' and can even get his hands on black market goods for the right price.